It is a well-known phenomenon that many metals, alloys and chemical compounds substantially lose all of their electrical resistance at temperatures near absolute zero. This phenomenon has highly advantageous applications when applied to electrical alternators or generators. To achieve the advantages resulting from superconductivity in the generator, and in particular, the electrical winding thereof, the winding must be operated below the critical temperature (above which the winding returns to its normal, resistive conducting state). The critical temperature itself is a function of conductor current density and magnetic field strength. In general, the lower the temperature, the greater the current density and magnetic field may be.
In the past, it has been proposed to operate generators in their superconducting mode by submerging them in a liquid helium pool to keep the temperature of the winding below its critical temperature.
Generally speaking, the construction of a generator or alternator for superconductive operation entails the provision of a generally cylindrical, gas-tight outer shell that rotates with the shaft of the rotor. The electrical winding (hereinafter "winding") is disposed interiorly of and spaced a slight distance inwardly from the shell. It too rotates with the shaft. A quantity of helium is placed inside the shell which is sufficient to fully submerge the winding in liquid helium when the generator rotates at its normal operating speed. At that point, the pool forms an inwardly facing liquid helium interface from which helium boils off into the gaseous center or core space of the rotor. Means must be provided to replenish the helium as it boils off and to keep the helium bath at a sufficiently low temperature so that the winding will remain below the critical temperature at all times.
Liquid helium is normally introduced into the shell interior via a central bore in one of the shafts. Helium vapor is normally withdrawn through a similar bore at either end of the rotor for recirculation through a conventional refrigeration system employing a compressor and a condenser.
A drawback of this type of construction is the fact that fresh liquid helium must constantly be transported radially outward towards the outer shell for cooling of the winding that is proximate the shell. Due to the relatively high rate of rotation of the shell, this entails a compression of the liquid helium which in turn raises its temperature. As a result, the temperature of the helium bath at the radially outermost point is higher than the temperature of the bath adjacent the helium bath interface. Yet, the lowest helium temperature is required in the area of the winding because it is the winding that must be kept below the critical temperature. Since the temperatures involved are very low, the critical temperature for a typical winding being normally in the vicinity of 4.5.degree. K., even slight temperature increases in the order of a fraction of a degree are most undesirable because they can cause the winding to intermittently return to its normal, i.e. non-superconducting state. Thus, it is apparent that superconducting generators operating in a liquid helium pool are of questionable reliability and, in fact, it is believed that no such generators have ever been built on a commercial scale.
Generally speaking, Roebuck-type refrigeration systems are also known though not for cooling generator rotors below their critical temperature for reasons that will be set forth in greater detail below. Briefly, in a Roebuck refrigeration device a conduit for the liquid cooling medium, e.g. for liquid helium is rotated at a relatively high rate and is given a crank or horseshoe shape. The medium to be cooled enters from one end of the pipe and it is compressed by the centrifugal force acting on the medium as the medium travels along the lateral portion of the pipe. During the centrifugal compression of the medium, the medium is cooled from the exterior of the pipe to withdraw from the medium sensible heat of compression (resulting from the increased pressure acting on the medium as it travels radially outward). When the "cooled" medium returns radially inward on the second leg of the crank-shaped pipe, its pressure decreases, it expands and thereby cools to below the temperature at which it was when it entered the pipe. For a further discussion of the principal and operation of a Roebuck refrigeration device see Cryogenic Engineering by R. B. Scott, D. Van Nostrand Co. Inc., Princeton, N.J. 1967, pages 38 and 39.
Incorporating a Roebuck-type refrigeration device in the rotor of a superconducting generator or alternator has been thought not feasible for the simple reason that the electrical winding of the rotor must be cooled to such an extent that the winding must be submerged in the cooling medium, e.g. the liquid helium. Since the winding could not be placed in a rotating, horseshoe-shaped pipe, a Roebuck-type refrigeration device appears to be inapplicable to supercooled generators.
In addition, even if the winding of a rotor could be placed within the horseshoe-shaped conduit of a Roebuck device, the cooling efficiency of such a device would be relatively low because of coolant vapors that are entrained in the liquid coolant. Such vapors cannot be removed from a closed loop and adversely affect the efficiency with which the coolant can absorb heat from the winding.
A further drawback with proposed prior art cooling systems for the operation of a generator rotor in its superconducting state is the fact that it is necessary to operate the gaseous core space in the shell into which liquid helium evaporates at a relatively low pressure in order to attain the necessary low operating temperature. Normally, that pressure is the vapor pressure of the cooling medium which is less than the ambient pressure; for helium it is about one-half atmosphere. In such a case, the liquid helium must be supplied at a like pressure, resulting in a vacuum condition in the intake conduit which can readily lead to air inclusions in the helium flow due to air leaks in the system. If such inclusions occur, slush forms rapidly which adversely affects the cooling flow and which, under extreme circumstances, can lead to a blockage of passages and a breakdown of the cooling system. Needless to say, that would have catastrophic consequences for the operation of the generator.